Thread chasers having the last tooth free of flank contact rearwardly of the thread crest cut thereby



June 18, 1963 J. w. KELSO 3,093,350

THREAD CHASERS HAVING THE LAST TQOTH FREE OF FLANK CONTACT REARWARDLY OF THE THREAD CREST CUT THEREBY Filed Oct. 30. 1959 5 Sheets-Sheet 1 Conventional Chaser Die T Horizon fol 1 15- B El .uznu. nsuoveo or METAL REMOVED BY T007. 0F CHASE TOOTH I "A 2 IIB an [IA *Firsi fully formed loom Tit an-E C!) as or 30 A S 1 Arc of finished eras! Arc of finished roof of pipe thread of MP. Mrnd mvmroe JOHN w. KELSO Attorney THEREBY June 18. 1963 J. w. KELSO THREAD CHASERS HAVING THE LAST TOOTH FREE OF FLANK CONTACT REARWARDLY OF THE THREAD CREST CUT Filed Oct. 30. 1959 5 Sheets-Sheet 2 M w E V m JOHN W. KELSO Attorney June 18, 1963 J. w. KELSO 3,093,350

THREAD CHASERS HAVING THE LAST TOOTH FREE OF FLANK CONTACT REARWARDLY OF THE THREAD CREST CUT THEREBY Filed Oct. 30. 1959 5 Sheets-Sheet 3 INVENTOR JOHN W KELSO AI/ omey United States Patent 3,093,850 THREAD CHASERS HAVING THE LAST TOOTH FREE OF FLANK CONTACT REARWARDLY OF THE THREAD CREST CUT THEREBY John W. Kelso, Dravosburg, Pa., assignor to United States Steel Corporation, a corporation of New Jersey Filed Oct. 30, 1959, Ser. No. 849,858 3 Claims. (Cl. 10-111) This invention relates to improvements in apparatus for cutting tapered threads on the ends of pipe, bar or the like.

A particular object of the invention is to provide improved chaser dies for such operations characterized by identical profiles. Such characteristic reduces die cost by simplifying die manufacture and greatly reduces die inventory. I

A further object is to provide a die having improved cutting characteristics whereby the number of chaser dies required to effect a full thread and the threading time are greatly reduced. 7

Another object is to provide a chaser die adapted to the use of sintered carbides and the like as the die material.

Other objects and adaptations of the invention will become apparent as the description proceeds, in' which:

FIGURE 1 is a partial section through'th'e threaded end of a pipe showing the profile of the No. l chaser of a set of conventional chaser dies and certain characteristics of the thread cut by such a set of conventional chaser dies;

FIGURE 2 is a schematic showing of the order, shape and certain other characteristics of the progressive cuts taken by the teeth of a set of conventional chaser dies during the formation of a single full thread;

FIGURE 3 is a partial elevation of a threading machine showing the chaser head thereof and a preferred arrangement of the improved chaser dies of the present invention therein;

FIGURE 4 is a partial IV-IV of FIGURE 3; 7

FIGURE 5 is similar to FIGURE 2 but showing the shape and sequence of cuts made by the improved chaser dies of the present invention in forming a single full thread; and

FIGURE 6 is an enlarged view showing preferred rake and clearance angles of the teeth of the improved dies.

My invention is primarily concerned with the threading of pipe 4 inches or more in diameter, commonly termed casing, tubing and drill pipe. A symmetrical tapered thread of 8, in some instances threads per inch, running out at a steeper angle than the angle of taper, is used for such product. Because of rigorous service conditions, dimensions, quality and other characteristics of thread are limited by strict specifications (see API Standard 5A March 1958). Fundamental dimensions of such section taken along the line threads are:

Taper (on Threads/inch diameter) Pitch Flank (inches! angle, deg.

loot) Depth of thread varies with the specified root and crest shape and ranges, in the case of 8 threads per inch, from 0.1083 inch sharp threads to 0.07125 inch for API round threads. The threads are usually run out at an angle of between 10 and In the art of cutting such threads it has been the practice heretofore to use a series of 6 to 12 chaser dies having the general profile indicated in FIGURE 1. Such dies are characterized by several truncated teeth 1, 2, 3 and 4 comprising a lead or runout followed by two or more fully formed teeth 5. The crestcutting portions 6 of all the teeth are fully formed and lie tangent to the major cone 7 of the finished thread; the fully formed root-cutting portions 8 of teeth 5 lie tangent to minor cone 9. by chamfering teeth 1, 2, 3, and 4 along is the half-apex angle of vanish cone 10 of the finished thread. Since the length of the lead teeth of each succeeding die must increase, the profiles of the dies comprising a conventional chaser set vary with their position in the set. Conventional chaser die sets produce a runout characterized by threads having fully formed crests but truncated roots. The full thread is cut from the crest downward by each succeeding tooth deepening and narrowing the cut made by preceding teeth. This action is depicted schematically in FIGURE 2 wherein for simplicity it is assumed that the thread has been cut by a single die of the form shown in FIGURE 1. Attention is. directed to the fact that with the exception of the first tooth, the crest-cutting porticns and n increasing length of the flanks of each succeeding tooth simply rub upon surfaces previously cut Conventional chaser di f tool steel and the of operating at higher than tool steel, and therefore at higher threading speeds, have been unsuccessful. I have found the of such carbide dies as well as a number of other limitations of the threading operations is largely attributablerto the aforementioned rubbing action inherent in the conventional chaser dies.

FIGURES 3 and 4 show a preferred embodiment of the present invention and serve to illustrate the principles thereof. In this embodiment, threading is accomplished by two chaser dies 11A and 11B mounted in a retractable chaser head 12 of a suitable threading machine. With the exception of simplificati on of the chaser head 12 to accommodate only two dies, the threading machine is con ventional and will not be described in detail.

As shown in FIGURE 3, die 11A is carried by the left half 12L of the head; die 11B, by the right half 12R. The two halves of the head are slidably mounted on a carriage 13 and each is urged into cutting contact with pipe P by a reversible hydraulically operated mechanism 14. Movement of each half head toward pipe P is limited by a downwardly depending bracket 15 carrying a roller 16 which bears upon a sine bar 17 fixedly attached to the main bed 18 of the machine. Carriage 13 is slidably mounted on bed 18 and moved longitudinally of the pipe by the lead screw (not shown) of the machine. Pipe P is supported and rotated by a chuck (not shown) carried by the drive head (not shown) of the machine. The gearing of the drive head and lead screw are selected as required by the pitch of the thread to be cut; the dimensi'ons and angularity of the sine bar by the degree of thread taper desired. a V a i 7 As shown i FIGURE 4, chasers 11A and nun-essentical in profile. Each has four cutting teeth, numbered 21, 22, 23, and 24 fromthe forward face 25 of the die. The teeth are spaced one pitch of finished thread apart and each comprises a fully formed, al enate, root-cutting portion 26 andyleading and following flank-cutting portions 27 and 28, respectively. The are j T at flankangles C and C with respect to a line perpendicular to axis 29 of pipe P. In the symmeli'icaI of the illustration, angles C and C are equal andequal to the half angle of the finished thread. Root-cutting portions 26 of all the teeth are arranged to lie tangent to a line 10 opening toward the forward face 25 at a lead angle A. Line 10 defines the vanish cone of the finished thread; angle A, the half-apex angle thereof. The following-fiank-cutting portion 28 of rearmost tooth 24 terthis tooth as required to cut a finished thread of desired height and taper. The latter is indicated by angle B which is the half-apex angle of the major and minor cones 7 and 9, respectively, of the terminated in a rounded heel portion 31 at its point of tangency with the lead flank of the finished thread. This must be done to avoid deleterious rubbing and consequent damage to the thread at this point.

Dies 11A and 11B are spaced 180 apart with 11A the No. l chaser of the pair, a half pitch ahead of 118. The dies are positioned in head 12 with their bases 32 in planes parallel to axis 29 of pipe P with chaser die 11B being shimrned, by spacer 33, toward axis 29 by an amount times the tangent of angle A divided by the number of dies comprising the set.

As indicated by the greatly reduced number of cutting teeth, the present invention contemplates a greatly increased cutting load per tooth as well as much higher threading speeds. Accordingly, the rate of heat generation at the cutting surfaces is enormously increased and dies 11A and 118 must be made of tungsten carbide or equivalent material capable of withstanding these more rigorous service conditions. Experience has shown that the carbide dies of my invention perform best if all cutting edges are lapped to a negative rake angle S of between 12 and 30. Further that length L of rootand crest-cutting portions 26 and 30, respectively, be no more than about 0.030 inch, with the balance of the portions lapped to provide a relief angle R of 3 to 7 as indicated in FIGURE 6. It is unnecessary to provide a helical lead to the teeth of the chasers of the present invention. Such lead, however, may be desirable in chasers for threading pipe of less than 4 inches in diameter.

The terms pitch, flank angle, vanish cone, major, and minor cone, etc. are used herein as defined in ASME Publication ASA B 1.7-1949, American Standard Nomenclature, Definitions and Letter Symbols for Screw Threads.

In operation, the pipe P is gripped and rotated by the drive head of i e machine and chaser head 12 is advanced onto the pipe by the lead screw of the machine. Dies 11A and 11B are maintained in proper taper-cutting position by the hydraulic mechanisms 14 forcing the halves 12R and 12L of the head into contact with sine bar 17; the sides of which diverge at the desired angle of taper B. Thus as head 12 advances, the bar 17 forces 12R and L apart and the dies cut a thread tapering at angle B and running out at angle A. Upon completion of a thread of desired length, the mechanism 14 is reversed to re tract the dies and the pipe discharged from the machine.

Cutting a heavily tapered thread requires pre-tapering of the pipe. This may be done externally of the threading machine or by means of suitable cutting tools mounted in the chaser head immediately ahead of the dies 11A and 113. For the reasons which will be subsequently explained, it is essential to the purposes of the present invention that pro-tapering leave a topping of not less than .002 nor greater than .018 inch of metal to be removed by the erest-cutting portion 30 of tooth 24 of die 118.

As illustrated in FIGURE the sequence of cutting is: tooth 21 of die 11A, tooth 21 of die 11B, tooth 22 of die 11A, etc. The amount and shape of the metal removed by each tooth is indicated by the shaded area associated with each tooth. It is apparent that tooth 24 of 118 effects a transition from runout to full thread form,

1 No satisfactory thread produced.

In addition to a remarkable decrease in without sacrifice of die life, produce a smoother, harder reduced tendency to seize in threading time the chasers of my invention thread having a considerable by lateral pressures a minimum of two chasers of about 0.008 inch, a depth of cut of about 0.009 inch per die. The teeth of such minimum of 0.0045 inch of metal at the flanks and the resulting thread would run out at an angle of about 10. It should be noted that with a topping of 0.008, a taper angle of 147, and a runout angle of 10", the first tooth of the first die (11A) of the pair would The latter however, is well within the 0.018 maximum previously specified.

The same thread can be cut with a series of three identical chasers of three teeth each, in which case, the nominal depth of cut per tooth would be 0.008 inch, removing a minimum of about 0.004 inch at the flanks. The resulting thread would run out at about 1356. The second die of such series of three identical chasers must be positioned Va x .125 x tan 1356 or 0.0103 inch toward pipe axis 29 and the third die thereof, an additional 0.0103 or a total of about .021 inch; the three dies are preferably spaced and /a pitch apart in the chaser head.

While I have described my invention as applied in the cutting of the round API threads, the principles thereof are applicable to cutting of other standard threads ineluding non-symmetrical threads, provided such threads have flank angles greater than .002 "1 4 COS .018

the latter limit being dictated by the aforementioned minimum and maximum depths of metal removal by any cutting surface of the dies.

Chaser dies embodying the principles outlined above can be used in any conventional threading machine comprising a chaser head adapted to receive and support the dies in threading position, means for supporting the pipe to be threaded, means for rotating the head or pipe, one relative the other, means for advancing either the head or the pipe longitudinal one of the other at a rate controlled in relation to the speed of rotation to produce finished threads of the desired pitch, and tapering means associated with the chaser head whereby the chaser dies are retracted transversely at a uniform rate during their thread-cutting engagement to produce a thread of a desired taper. Such machines are well known in the art and several difierent designs are available. Of course, as previously mentioned, a slight modification of the chaser head must be made since the present invention contemplates use of sets of but two or three dies.

While I have shown and described certain specific em bodiments of my invention, 1 do not wish to be limited exactly thereto except as defined in the appended claims.

I claim:

1. Carbide chaser dies adapted to be mounted in a retractable chaser head to cut a fully formed tapered thread running out at a steeper taper than the fully formed thread, said chaser dies having forward and rearward ends at least three and not more than four carbide thread cutting teeth spaced apart one pitch of the finished thread cut thereby, each of said teeth having a fully formed root-cutting portion tangent to the vanish cone of the said finished thread, a leading flank-cutting portion and a following flank-outing portion having flank angles equal respectively to the leading and following flank angles of the finished thread, said flank angles being greater than a EB COS .018 the rearmost tooth only having a crest-cutting portion disposed rearwardly of its following flank-cutting portion, said chaser being substantially free of flank contact rearwardly of the thread crest cut thereby, said crest-cutting and rootcutting portions of said rearmost tooth being tangent to the major and minor cones respectively of the said finished thread.

2. Carbide chaser dies adapted to be mounted in a retractabie chaser head to cut a fully formed tapered symmetrical thread running out at a steeper taper than the fully formed thread, said chaser dies having forward and rearward ends and at least three and not more than four thread cutting teeth spaced apart one pitch of the finished thread cut thereby, each of said teeth having a fully formed root-cutting portion tangent to the vanish cone of the finished thread, a leading flank-cutting portion and a following flank-cutting portion, the rearmost tooth having a crest-cutting portion extending rearwardly from its following flank-cutting portion, said chaser being substantially free of flank contact rearwardly of the thread crest cut thereby, said crest-cutting and root-cutting portions of said rearmost tooth being tangent to the major and minor cones respectively of the finished thread, the following flank-cutting portion and the leading flankcutting portion of all the teeth having a flank angle equal to the half angle of the finished thread, said half angle being greater than 3. In apparatus for cutting a fully formed tapered thread of the desired pitch and depth having a runout of steeper taper than the taper of the fully formed thread portion on a cylindrical workpiece including a die head having a forward face adapted to receive the workpiece the combination therewith of a series of at least two but not more than three identical carbide chaser dies, each of said chaser dies having forward and rearward ends and at least three and not more than four carbide thread cutting teeth spaced apart one pitch of the finished thread cut thereby, each of said teeth having a fully formed root-cutting portion tangent to the vanish cone of said finished thread, a leading flank-cutting portion and a following flank-cutting portion, the rearmost tooth having a crest-cutting portion disposed rearwardly of its following flank-cutting portion, said chaser being substantially free of flank contact rearwardly of the thread crest cut thereby, said crest and root-cutting portions of said rearmost tooth being tangent to the major and minor cones respectively of the said finished thread, the following and the leading flank-cutting portions of all of said teeth having flank angles equal to those of said finished thread, said flank angles being greater than each of said chaser dies being mounted in said head with its teeth extending inwardly therefrom and with each succeeding die positioned inwardly an additional amount equal to the depth of the said finished thread divided by the number of dies in the said series of chaser dies and with each succeeding die being displaced angulariy and rearwardly, said rearward displacement being an amount equal to the pitch of the said finished thread multiplied by the ratio of the angular displacement to 360.

References Cited in the file of this patent UNITED STATES PATENTS 1,762,652 Cox June 10, 1930 1,839,684 Lawrenz Jan. 5, 1932 1,911,598 Ashmun May 30, 1933 2,067,593 Benninghotf Ian. 12, 1937 2,744,269 Kerr May 8, 1956 FOREIGN PATENTS 154,310 Sweden May 2, 1956 

1. CARBIDE CHASER DIES ADAPTED TO BE MOUNTED IN A RETRACTABLE CHASER HEAD TO CUT A FULLY FORMED TAPERED THREAD RUNNING OUT AT A STEEPER TAPER THAN THE FULLY FORMED THREAD, SAID CHASER DIES HAVING FORWARD AND REARWARD ENDS AT LEAST THREEE AND NOT MORE THAN FOUR CARBIDE THREAD CUTTING TEETH SPACED APART ONE PITCH OF THE FINISHED THREAD CUT THEREBY, EACH OF SAID TEETH HAVING A FULLY FORMED ROOT-CUTTING PORTION TANGENT TO THE VANISH CONE OF THE SAID FINISHED THREAD, A LEADING FLANK-CUTTING PORTION AND A FOLLOWING FLANK-CUTTING PORTION HAVING FLANK ANGLES EQUAL RESPECTIVELY TO THE LEADING AND FOLLOWING FLANK ANGLES OF THE FINISHED THREAD, SAID FLANK ANGLES BEING GREATER THAN 